While it could be done, I don't see the entire Silmarillion story appealing to audiences. A bit too Creationist at the beginning, it would offend religious audiences. It is a book without a clear ending.

The only start-to-finish character is Melkor, the antagonist. It is better for movies to have the protagonist to focus on from start to finish.

I can see parts of it as a movie, but not all of it. Turin, sure. A Beren and Luthien love story would be possible. I could also see a mini-series or TV show about the fall of the sons of Feanor.